It is generally well known in the art, that the access doors on passenger transit railway vehicles, such as subway cars and the like, are provided with an automatic opening and closing mechanism which is controlled by the operator of such vehicle. It is also well known that such access doors are normally provided with an automatic locking mechanism that will lock such access doors in the closed position when the vehicle is in motion to safeguard the passengers during normal operation. These access doors, however, must also be provided with a mechanism that will permit an override or by-passing of the automatic locking mechanism to enable the operator or a passenger to manually open such access doors in the event of an emergency.
Typically, such an access door or pair of doors are operated by either a hydraulic or pneumatic cylinder which is connected to a linkage mechanism adapted to open and close the door or doors in response to the reciprocating action of the hydraulic or pneumatic cylinder rod. In most operating situations, the doors are locked in place in the closed position by maintaining a positive pressure within the hydraulic or pneumatic cylinders while the doors are in the closed position. When the doors are automatically opened by the vehicle operator, however, the hydraulic or pneumatic cylinders are activated in the opposite direction, thereby releasing the positive pressure within the cylinder which maintains the access doors in their locked position.
The emergency release device, on the other hand, is usually some type of mechanical system that will release an access door from the linkage system driven by the hydraulic or pneumatic cylinder, thereby permitting a passenger, for example, to force such access door open even though a positive pressure is being maintained within the hydraulic or pneumatic cylinder. In the event of a pressure loss in the hydraulic or pneumatic cylinder system, however, the access doors will not be locked in the closed position, thereby creating a hazardous condition.